Toggle your cursor over to the bottom left + icon, and click Add Conversations. Select the conversation or full collection you'd like to import highlights from. Click Submit.
Removing conversations and highlights from your catalog:
To remove highlights from your catalog, toggle over to the bottom left + icon and select Delete Conversations. From here you can select the name of a conversation you'd like to remove. Note: Removing a conversation from your catalog will also remove (and delete all tags) from all of the highlights from that conversation.